Problem: What number could replace $k$ below? $\dfrac{2}{10} = \dfrac{20}{k}$
The fraction on the left represents 2 out of 10 slices of a rectangular pizza. How many total slices would we need if we want the same amount of pizza in 20 slices? We would need to cut the pizza into 100 slices. $\dfrac{2}{10} = \dfrac{20}{100}$ and so the answer is $100$ Another way to get the answer is to multiply by $\dfrac{10}{10}$ $\dfrac{10}{10} = \dfrac{1}{1} = 1$ so really we are multiplying by 1. The final equation is: $\dfrac{2}{10} \times \dfrac{10}{10} = \dfrac{20}{100} $ so our answer is $100$.